One of the buildings looks a little transparent, and there's an icon over it. What does it mean?

If it's a road icon, you need to build a road leading to the object.
In case an icon has a "!" on it, it means that construction of the building is finished, but you need additional materials to complete its upgrading. Press the object to see them, and tap on a necessary one to find out the ways it can be obtained.
If it's a green arrow icon, the building can be upgraded.